An important problem in the biology of mammalian cell growth and differentiation concerns the regulation of messenger RNA (mRNA) biogenesis and its translation. Since the mRNA molecules appear to be associated with specific proteins through most of their life history in the cell, we studied the properties of mRNA bound proteins (mRNP) which contact inhibited and serum induced 3T3 cells, as an example of contrasting states of growth. The task now is to map the protein binding sites on specific mRNA, since similar proteins are found associated with a variety of mRNA species, and they may recognize common structural features present in most eukaryotic mRNA. The structural domains within the mRNA sequences recognized by the proteins, will be determined on the basis of the "nuclease protected" sequences of mRNA, labeled in vitro at 5'-termini with 32 pATP and T4 polynucleotide kinase, and cleaved by base specific nucleases and chemical probes. The nucleotide sequence of the end-labeled mRNA molecules is determined by base specific ribonuclease cleavages, and the "structure" is determined on the basis of single-strand specific S1 or T1 nucleases and the double-strand specific cobra venom nuclease V1 digestion in non-denaturing conditions. As an example of poly(A) lacking mRNA and the role of proteins in its translational regulation, we will focus on the cloned mouse histone H4 gene, expressed in recombinant SV40 virion infected monkey cells. These studies would reveal the specific structural features of a poly(A) lacking mRNA, that are recognized by messenger RNA binding proteins; most importantly, in this case, those features of histone mRNA-protein interactions that are specifically recognized in the regulation of histone gene expression. An important aspect of the proposed studies is the emphasis on characterizing the mRNA-protein interactions in specific mRNA. Rabbit globin mRNP studies could lead to further experimental manipulations of mRNP proteins in the translational regulation of mRNA in reconstituted systems; and the cloned mouse H4 mRNP studies on the translational feedback regulation of histone gene expression with respect to DNA replication. These studies will, therefore, aid in our understanding of cell growth regulation and neo-neoplasia.